This invention relates to high resolution intravascular imaging and more particularly to intraluminal ultrasound imaging and techniques for enhancing image quality.
In intraluminal ultrasound imaging, the production of high resolution images of vessel wall structures requires imaging at high ultrasound frequencies. Vectors are created by directing focussed ultrasonic pressure waves radially from a transducer in a catheter and collecting echoes at the same transducer from the target area. A plurality of radial vectors from the rotated transducer comprises an image frame. The backscatter from blood cells in such an image is a significant problem in high frequency intraluminal ultrasound imaging, since the scattering of ultrasound from blood cells is proportional to the fourth power of the frequency such that the higher the ultrasound frequency the more pronounced is the backscatter from blood. As a result, echoes from blood degrade the lumen-to-vessel wall contrast.
It is desirable to provide imaging over a broad range of frequencies, especially higher ultrasonic frequencies. Therefore, echoes in the ultrasound image due to backscatter from blood must either be removed or suppressed to a level at which wall structures can be distinguished from blood. Since blood is typically in motion relative to the image of interest, an individual frame will contain speckle due to the interference of blood constituents. One technique for enhancing image quality is to average successive image frames thereby to smooth out the impact of speckle (that is, the irregular pattern of backscatter from blood) in the intraluminal ultrasound images. However, such frame averaging is not effective in reducing the mean echo amplitude from the region of blood flow, and it cannot totally remove blood echoes from the image. What is needed is a mechanism for encoding or encrypting blood-induced echoes to allow a display of intraluminal ultrasound images free of blood-induced echoes.
Background information on the subject of intravascular ultrasonography is found in U.S. Pat. No. 4,794,931 and related U.S. Pat. No. 5,000,185.